1Table of contents
2=================
3 4Last updated: 20 December 2005
5 6Contents
7========
8 9- Introduction
10- Devices not appearing
11- Finding patch that caused a bug
12-- Finding using git-bisect
13-- Finding it the old way
14- Fixing the bug
15 16Introduction
17============
18 19Always try the latest kernel from kernel.org and build from source. If you are
20not confident in doing that please report the bug to your distribution vendor
21instead of to a kernel developer.
22 23Finding bugs is not always easy. Have a go though. If you can't find it don't
24give up. Report as much as you have found to the relevant maintainer. See
25MAINTAINERS for who that is for the subsystem you have worked on.
26 27Before you submit a bug report read REPORTING-BUGS.
28 29Devices not appearing
30=====================
31 32Often this is caused by udev. Check that first before blaming it on the
33kernel.
34 35Finding patch that caused a bug
36===============================
37 38 39 40Finding using git-bisect
41------------------------
42 43Using the provided tools with git makes finding bugs easy provided the bug is
44reproducible.
45 46Steps to do it:
47- start using git for the kernel source
48- read the man page for git-bisect
49- have fun
50 51Finding it the old way
52----------------------
53 54[Sat Mar 2 10:32:33 PST 1996 KERNEL_BUG-HOWTO lm@sgi.com (Larry McVoy)]
55 56This is how to track down a bug if you know nothing about kernel hacking.
57It's a brute force approach but it works pretty well.
58 59You need:
60 61 . A reproducible bug - it has to happen predictably (sorry)
62 . All the kernel tar files from a revision that worked to the
63 revision that doesn't
64 65You will then do:
66 67 . Rebuild a revision that you believe works, install, and verify that.
68 . Do a binary search over the kernels to figure out which one
69 introduced the bug. I.e., suppose 1.3.28 didn't have the bug, but
70 you know that 1.3.69 does. Pick a kernel in the middle and build
71 that, like 1.3.50. Build & test; if it works, pick the mid point
72 between .50 and .69, else the mid point between .28 and .50.
73 . You'll narrow it down to the kernel that introduced the bug. You
74 can probably do better than this but it gets tricky.
75 76 . Narrow it down to a subdirectory
77 78 - Copy kernel that works into "test". Let's say that 3.62 works,
79 but 3.63 doesn't. So you diff -r those two kernels and come
80 up with a list of directories that changed. For each of those
81 directories:
82 83 Copy the non-working directory next to the working directory
84 as "dir.63".
85 One directory at time, try moving the working directory to
86 "dir.62" and mv dir.63 dir"time, try
87 88 mv dir dir.62
89 mv dir.63 dir
90 find dir -name '*.[oa]' -print | xargs rm -f
91 92 And then rebuild and retest. Assuming that all related
93 changes were contained in the sub directory, this should
94 isolate the change to a directory.
95 96 Problems: changes in header files may have occurred; I've
97 found in my case that they were self explanatory - you may
98 or may not want to give up when that happens.
99 100 . Narrow it down to a file
101 102 - You can apply the same technique to each file in the directory,
103 hoping that the changes in that file are self contained.
104 105 . Narrow it down to a routine
106 107 - You can take the old file and the new file and manually create
108 a merged file that has
109 110 #ifdef VER62
111 routine()
112 {
113 ...
114 }
115 #else
116 routine()
117 {
118 ...
119 }
120 #endif
121 122 And then walk through that file, one routine at a time and
123 prefix it with
124 125 #define VER62
126 /* both routines here */
127 #undef VER62
128 129 Then recompile, retest, move the ifdefs until you find the one
130 that makes the difference.
131 132Finally, you take all the info that you have, kernel revisions, bug
133description, the extent to which you have narrowed it down, and pass
134that off to whomever you believe is the maintainer of that section.
135A post to linux.dev.kernel isn't such a bad idea if you've done some
136work to narrow it down.
137 138If you get it down to a routine, you'll probably get a fix in 24 hours.
139 140My apologies to Linus and the other kernel hackers for describing this
141brute force approach, it's hardly what a kernel hacker would do. However,
142it does work and it lets non-hackers help fix bugs. And it is cool
143because Linux snapshots will let you do this - something that you can't
144do with vendor supplied releases.
145 146Fixing the bug
147==============
148 149Nobody is going to tell you how to fix bugs. Seriously. You need to work it
150out. But below are some hints on how to use the tools.
151 152To debug a kernel, use objdump and look for the hex offset from the crash
153output to find the valid line of code/assembler. Without debug symbols, you
154will see the assembler code for the routine shown, but if your kernel has
155debug symbols the C code will also be available. (Debug symbols can be enabled
156in the kernel hacking menu of the menu configuration.) For example:
157 158 objdump -r -S -l --disassemble net/dccp/ipv4.o
159 160NB.: you need to be at the top level of the kernel tree for this to pick up
161your C files.
162 163If you don't have access to the code you can also debug on some crash dumps
164e.g. crash dump output as shown by Dave Miller.
165 166> EIP is at ip_queue_xmit+0x14/0x4c0
167> ...
168> Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00
169> 00 00 55 57 56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08
170> <8b> 83 3c 01 00 00 89 44 24 14 8b 45 28 85 c0 89 44 24 18 0f 85
171>
172> Put the bytes into a "foo.s" file like this:
173>
174> .text
175> .globl foo
176> foo:
177> .byte .... /* bytes from Code: part of OOPS dump */
178>
179> Compile it with "gcc -c -o foo.o foo.s" then look at the output of
180> "objdump --disassemble foo.o".
181>
182> Output:
183>
184> ip_queue_xmit:
185> push %ebp
186> push %edi
187> push %esi
188> push %ebx
189> sub $0xbc, %esp
190> mov 0xd0(%esp), %ebp ! %ebp = arg0 (skb)
191> mov 0x8(%ebp), %ebx ! %ebx = skb->sk
192> mov 0x13c(%ebx), %eax ! %eax = inet_sk(sk)->opt
193 194In addition, you can use GDB to figure out the exact file and line
195number of the OOPS from the vmlinux file. If you have
196CONFIG_DEBUG_INFO enabled, you can simply copy the EIP value from the
197OOPS:
198 199 EIP: 0060:[<c021e50e>] Not tainted VLI
200 201And use GDB to translate that to human-readable form:
202 203 gdb vmlinux
204 (gdb) l *0xc021e50e
205 206If you don't have CONFIG_DEBUG_INFO enabled, you use the function
207offset from the OOPS:
208 209 EIP is at vt_ioctl+0xda8/0x1482
210 211And recompile the kernel with CONFIG_DEBUG_INFO enabled:
212 213 make vmlinux
214 gdb vmlinux
215 (gdb) p vt_ioctl
216 (gdb) l *(0x<address of vt_ioctl> + 0xda8)
217or, as one command
218 (gdb) l *(vt_ioctl + 0xda8)
219 220If you have a call trace, such as :-
221>Call Trace:
222> [<ffffffff8802c8e9>] :jbd:log_wait_commit+0xa3/0xf5
223> [<ffffffff810482d9>] autoremove_wake_function+0x0/0x2e
224> [<ffffffff8802770b>] :jbd:journal_stop+0x1be/0x1ee
225> ...
226this shows the problem in the :jbd: module. You can load that module in gdb
227and list the relevant code.
228 gdb fs/jbd/jbd.ko
229 (gdb) p log_wait_commit
230 (gdb) l *(0x<address> + 0xa3)
231or
232 (gdb) l *(log_wait_commit + 0xa3)
233 234 235Another very useful option of the Kernel Hacking section in menuconfig is
236Debug memory allocations. This will help you see whether data has been
237initialised and not set before use etc. To see the values that get assigned
238with this look at mm/slab.c and search for POISON_INUSE. When using this an
239Oops will often show the poisoned data instead of zero which is the default.
240 241Once you have worked out a fix please submit it upstream. After all open
242source is about sharing what you do and don't you want to be recognised for
243your genius?
244 245Please do read Documentation/SubmittingPatches though to help your code get
246accepted.
247